Administration of intravenous fluids to a patient is well known in the art. Typically a solution such as saline, glucose or electrolyte in a glass or flexible container is fed to a patient's venous access site via a length of flexible plastic tubing, called an administration set. The rate of flow of the fluid is controlled by a roller clamp which is adjusted to restrict the flow lumen of the set until the desired flow rate is obtained.
In some cases an intravenous administration set up includes a second container of fluid, usually containing a medication such as an antibiotic. In that instance the administration set includes three branches connected by a Y-site. One branch is connected to the first container, one branch is connected to the second container, and the third branch is connected to the patient access site. When two containers are used, one of the containers is hung at a higher elevation than the other. The higher head pressure that results from the higher elevation causes the higher container to empty before the lower one. A check valve is provided in the Y-site branch leading to the lower container so that fluid will flow from the higher container to the patient, and not to the lower container.
This two container arrangement is typically used when a patient is scheduled to receive an intermittently administered medication. In that instance the line from the upper container to the Y-site is clamped off until the time when the medication is to be administered. Until then, fluid from the lower container is continually supplied to the patient at the desired rate. When the medication is to be administered the clamp to the upper container is opened. The greater head pressure in the upper container causes the fluid to flow from that container, and automatically cuts off the flow from the lower container. When the upper container is emptied, flow from the lower container will automatically resume to the patient.
Flow from the two containers to the patient may also be regulated by means other than a roller clamp. It is becoming more and more common to use an electronically controlled pump.
If a single pump is positioned between the Y-site and the patient access site in a two container set up, its use does not affect the preferential flow from the higher container. The pump will control the rate of flow to the patient regardless of which source the fluid is coming from.
One type of pump that is used for intravenous fluid administration has a pump cassette which is positioned in the fluid flow path between the patient and the fluid source. The cassette has a variable volume pumping chamber connected via tubing at its inlet side to tubing that is connected to the fluid source, and at its outlet side to tubing that is connected to the patient's access site.
In operation the pump cassette is positioned in a pump which has an operating mechanism to force fluid from the source to the patient by expanding and contracting the volume of the pumping chamber. Expansion draws fluid into the chamber from the fluid source, and contraction forces the fluid from the chamber to the patient. An example of such a pump and cassette is shown in U.S. Patent Application No. 07/411,789 filed Sept. 25, 1989 and assigned to the assignee of the present invention, the disclosure of which is incorporated herein.
A characteristic of intravenous pumps of this type is that when the pump chamber is being filled, the expansion of its volume causes a pressure drop to occur upstream of the chamber. This drop in pressure is not a problem when only a single fluid source is employed. However, when there are two fluid sources with one at a higher elevation than the other, the pressure drop can have the effect of lowering the pressure at the check valve to a value that is lower than the pressure upstream of the valve. This opens the check valve and permits fluid to flow from the primary (lower) container.
In order to overcome this problem it has been proposed in the prior art to isolate the effect of the pressure drop by interposing a restriction in the flow path between the pump and the fluid sources, or by using an air chamber in the same location. Both of these have disadvantages. The restriction also limits the flow rate from the fluid source to the pumping chamber and thus affects speed and accuracy of the pump. The air chamber is undesirable because it is position sensitive and suffers from the drawbacks of introducing air into an intravenous administration line.